The present invention generally relates to a system for detecting modulated laser signals and, in particular, relates to one such system having means for storing the occurrences of a plurality of different pulse intervals in accordance with at least one preselected characteristic of the different pulse intervals and outputting a signal when a weighted sum of the number of different pulse intervals counted exceeds a preselected threshold number.
Many modem weapon systems employ modulated laser signals to locate and/or aim a weapon at a target. As used herein the term "modulated" includes laser signals that are generated either by pulsing a laser on and off of by chopping the output of a continuous wave laser. Ordinarily, the modulated laser signal generated by pulsing the laser on and off will result in a signal having relatively short high intensity pulses spaced apart by long intervals. The modulated laser signal generated by chopping a continuous wave laser usually has relatively longer pulses than that of the pulsed laser.
It is therefore quite important to be able to detect the presence of modulated laser signals reliably and rapidly.
Presently, modulated laser signals are detected by systems that have a single channel. The system counts the temporal interval between successive detected pulses and stores, or bins, the information according to the interval between each pulse. When the number of occurrences of any given pulse interval reaches a preselected threshold the system indicates the presence of a modulated laser signal.
Such a system has a number of drawbacks, the first of which is the fact that only the occurrence of one specific pulse interval, i.e., information from only one bin, is used to determine the presence of a modulated signal while information from other bins is not used to make a determination. This approach resulting in an unnecessarily low system sensitivity. For example, if a signal is detected, i.e., the signal has sufficient intensity above the background noise, the interval counter begins counting the interval. The next signal detected stops the counter for the first interval and starts a new interval count, the first interval being stored in a bin for that specific time interval and the occurrence count for that bin is incremented by one. When many pulses have occurred, in fact, there may have been regular pulse intervals undetected due to background noise or the like during the first counted interval. Present systems are unable to correct for or make use of this information.
Further, modem systems cannot account for common slightly changing pulse intervals originating from the same source. Modern modulated laser signal detection systems store such signals in different pulse interval bins. As a result, the signal presence threshold would take an excessive time to be reached.
Still further, since such conventional systems are single channel systems, all of the background noise reaching the system input is placed into the same bin as the signal.
In addition, present systems operate using only a single threshold regardless of the signal strength received. Hence, since the sensitivity of the system must be set to detect the lowest possible source intensity, indication of a relatively high intensity source may be delayed.
Further still, present systems, being single channel systems can not make use of known threat characteristics nor can they distinguish between pulsed and chopped continuous wave laser signals. Finally, present systems generally use a relatively large field of view. This results in a high noise input if the sun is anywhere in the field of view.
Consequently, it is readily recognized that a system for detecting modulated laser signals that overcomes these drawbacks is highly desirable.